Drilling method using high density, high strength, acid soluble pseudo-crosslinked, lost circulation preventative material

ABSTRACT

A formulation for use as a lost circulation preventive material is a cement-forming aqueous fluid comprising water, a viscoelastic surfactant (VES), a monovalent or multivalent salt, a magnesium powder, a retarder, a weighting material, and a dispersant. The formulation is used in a method of drilling into a subterranean formation that includes introducing into a wellbore passing at least partially through the subterranean formation the cement-forming aqueous fluid, and further increasing the viscosity of the aqueous fluid with the VES; where the monovalent salt is present in an amount effective to pseudo-crosslink the elongated VES micelles to further increase the viscosity of fluid. The formulation further forms a cement by reacting the magnesium powder and the water which reaction is retarded by the retarder. The water may be saline water. When the fluid density is greater than 14 pounds per gallon, a dispersant is required, such as a sulfonated copolymer.

REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent application Ser. No. 14/834,779 filed Aug. 25, 2015, entitled HIGH DENSITY, HIGH STRENGTH, ACID SOLUBLE PSEUDO-CROSSLINKED LOST CIRCULATION PREVENTATIVE MATERIAL, issued ______ as U.S. Pat. No. ______, which in turn is a continuation-in-part of U.S. patent application Ser. No. 14/729,567, filed Jun. 3, 2015, entitled HIGH STRENGTH, OPERATIONALLY ROBUST LOST CIRCULATION PREVENTATIVE PSEUDO-CROSSLINKED MATERIAL, all of which are incorporated herein by reference in its entirety.

TECHNICAL FIELD

The present invention relates to methods of drilling through non-reservoir and reservoir sections of subterranean formations during hydrocarbon recovery operations, and more particularly relates, in one non-limiting embodiment, to using cement-forming aqueous fluids of relatively high density that contain components that inhibit or prevent fluid loss into the subterranean formation.

TECHNICAL BACKGROUND

Drilling fluids are categorized into water-based mud and oil-based mud. Drilling fluids are used to drill long horizontal and vertical sections of non-reservoir portions of a borehole. Water based drilling fluids designed with water, polymer needed to increase viscosity for carrying the cuttings and for fluid loss control, monovalent and multivalent salts for shale inhibition, different bridging material and weighting materials (barium sulfate, micronized barite, hausmannite, ilmenite, manganese tetroxide, hematite, and other fine, dense particles) for desired mud weight. Large amounts of weighting agents are often necessary to balance the mud weight against formation pressure. In heavily weighted fluids (greater than about 14 pounds per gallon up to about 20 pounds per gallon), large amounts of weighting agents make the dispersibility of other fluid components, particularly fluid-loss additives, difficult.

Drill-in fluids are special fluids designed exclusively for drilling through the reservoir section of a subterranean formation. The reasons for using specially designed drilling fluids include, but are not necessarily limited to (1) to drill the reservoir zone successfully, which is often a long, horizontal drain hole, (2) to minimize damage of the near-wellbore region and maximize the eventual production of exposed zones, and (3) to facilitate the necessary well completion. Well completion may include complicated procedures. Typically, drill-in fluids may resemble completion fluids. Drill-in fluids may be brines containing only selected solids of appropriate particle size ranges (for instance, salt crystals or calcium carbonate) and polymers. Usually, additives needed for filtration control and cuttings carrying are present in a drill-in fluid. As noted, drill-in fluids may contain filtration control additives to inhibit or prevent loss of the drill-in fluid into vugular zones of the permeable formation. Fluid loss involves the undesired leakage of the liquid phase of drill-in fluid containing solid particles and complete fluid losses into the formation matrix without any return. The resulting buildup of solid material or filter cake against the borehole wall may be undesirable, as may be the penetration of the filter cake into the formation. The removal of filter cake, which sometimes must be done by force, may often result in irreparable physical damage to the near-wellbore region of the reservoir. Fluid-loss additives are used to control the process and avoid potential damage of the reservoir, particularly in the near-wellbore region. Specially designed fluids may be used to be placed next to the reservoir and form a seal against fluid loss. This fluid may be different than the drill-in fluid and is often referred to as a “sealing or lost circulation pill”.

Subterranean formations having naturally occurring fractures present a problem because the fractures exacerbate undesired leakage of the liquid portion of the drill-in fluid into the formation. Thus, lost circulation fluid may be a major challenge when drilling through such naturally fractured carbonate, sandstone formations, etc.

Some lost circulation fluids are gelled, such as by gelling polymers and optionally crosslinking the resulting polymers. However, it is important to avoid premature gelation, and also important for the finally gelled material to have sufficient viscosity and strength to achieve the goal of inhibiting or preventing fluid loss. Current commercial offerings have insufficient operational robustness; examples including, but not necessarily limited to, commercial lost circulation material (LCM) technology based on polymer resin sealing or thixotropic slurry or high fluid loss pill.

It would thus be desirable to discover a drill-in fluid or a sealing pill, a drilling fluid or other fluid which would have relatively low viscosity in the drilling pipe and high dispersibility in heavily weighted (greater than about 14 pounds per gallon up to about 20 pounds per gallon) fluids but which would shortly after leaving the drill bit increase in viscosity and inhibit or prevent fluid leak-off into the formation.

SUMMARY

There is provided in one non-restrictive version, a method of drilling into a highly pressurized subterranean formation that requires a fluid density of greater than 14 pounds per gallon involving introducing into a wellbore passing at least partially through the subterranean formation a cement-forming aqueous fluid including a dispersant. The cement-forming aqueous fluid includes water, at least one viscoelastic surfactant (VES), at least one monovalent or multivalent salt, at least one magnesium powder comprising 30-80 wt % MgO and greater than 20 wt % dolomite, at least one retarder, at least one weighting material, and a dispersant. The method further involves increasing the viscosity of the cement-forming aqueous fluid by the action of the at least one VES forming elongated micelles, where the at least one monovalent salt is present in an amount effective to pseudo-crosslink the elongated VES micelles to further increase the viscosity of the aqueous fluid. Additionally, the method involves forming a cement by reacting at least one magnesium powder and the water, where the forming of cement is retarded from that which would otherwise occur but for the presence of the retarder. The method further involves inhibiting fluid loss of the fluid into the formation by the combined action of the pseudo-crosslinked VES micelles and the MgO-based cement.

There is also provided, in another non-limiting form, a cement-forming aqueous fluid having a density greater than 14 pounds per gallon for use in inhibiting the fluid loss of the cement-forming aqueous fluid into a subterranean formation, where the aqueous fluid includes water, at least one viscoelastic surfactant (VES), at least one monovalent or multivalent salt, at least one magnesium powder comprising 30-80 wt % MgO and greater than 20 wt % dolomite, at least one retarder, at least one weighting agent, and a dispersant. Optionally, a defoamer is included.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a photograph of a VES aqueous fluid containing MgO-based sealant powder in the blender with no dispersant;

FIG. 1B is a photograph of the VES aqueous fluid containing MgO-based sealant powder in the blender of FIG. 1A during dispersant addition;

FIG. 1C is a photograph of the VES aqueous fluid containing MgO-based sealant powder in the blender of FIG. 1B with dispersant during blending showing that a vortex opens after dispersant addition;

FIG. 1D is a photograph of the final set loss control material (LCM) plug made from the components mixed in FIGS. 1A-1C after being cured and removed from a mold;

FIG. 2A is a photograph of a crosslinked VES aqueous fluid containing sealant powder being poured showing its relatively thick viscosity;

FIG. 2B is a photograph of a VES aqueous fluid comparable fluid to that of FIG. 2A containing sealant powder that is not crosslinked being poured showing its relatively thin viscosity;

FIG. 3 is a thickening time chart for 15 ppg (1.8 kg/L) slurry without a weighting agent at 150° F. (66° C.);

FIG. 4 is a compressive strength chart for 15 ppg (1.8 kg/L) slurry without a weighting agent at 150° F. (66° C.);

FIG. 5 is a thickening time chart for a 16 ppg (1.9 kg/L) slurry with a manganese-based weighting agent at 200° F. (93° C.);

FIG. 6 is a chart of compressive strength for a 16 ppg (1.9 kg/L) slurry with a manganese-based weighting agent at 200° F. (93° C.);

FIG. 7 is a chart of a viscosity comparison of fluid by HPHT 5550 Viscometer with and without VES, and

FIG. 8 is a photograph of a solid set LCM for 15 ppg (1.8 kg/L) slurry after thickening time test.

DETAILED DESCRIPTION

A new formulation for use as a loss circulation solution has been discovered. A differentiation of the new formulation and its method of use involves the incorporation of viscoelastic surfactants (VESs) or similar chemistry that enables the formulation to be pumpable and have its gelation deferred until it is in right place in the subterranean formation. The aqueous fluid formulation contains at least one viscoelastic surfactant or similar chemistry with one or more soluble monovalent salts and/or brine and/or one or more multivalent salt or brine. In one non-limiting embodiment the formulations herein involve cement-forming aqueous fluids having a density greater than 14 pounds per gallon (ppg), alternatively those having a density greater than 14 ppg (1.7 kg/L) to about 20 ppg (2.4 kg/L); and in another non-restrictive version greater than 14 ppg (1.7 kg/L) to about 16 ppg (1.9 kg/L).

For lost circulation prevention or inhibition, it is important to avoid premature gelation but also for the gelled material to have sufficient viscosity and strength. As noted, current commercial offerings have insufficient operational robustness. The method and composition described herein could provide a better operating window, specifically the formulation has at least two additional advantages —(a) it could be formulated with saline water such as sea water and (b) it could be cleaned up post-job to recover a pre-job reservoir state. Saline water is defined herein to include, but not necessarily be limited to, sea water, produced water, synthetic sea water, formation water, salt water, brine, and the like and mixtures thereof. It has been discovered that the use of VES, with or without saline water, builds enough pumpable viscosity that carries a powder mixture of active ingredients, in non-limiting examples a magnesium based powder and borate powder. The choice of magnesium based powder is also important to the functioning of this formulation. The formulation does not setup prematurely in ambient conditions. The active mixture may be pumped to the zones in the reservoir and non-reservoir that are subject to fluid loss. Upon being heated to the wellbore temperature, it sets up as a cementitious plug with adequate strength to prevent the fluid losses. At the end of the operation, if required, the set solid material could be dissolved with acid flush. The slurry can be designed to have density in range of 10 ppg to 16 ppg (1.2 to 1.9 kg/L) and higher. The setting time of the plug is controlled by a boron-based retarder to about 1 to about 3 hours and more up independently to temperature from 70° F. (21° C.) and up to 230° F. (110° C.); alternatively forming the cement is triggered by a temperature between about 90° F. (32° C.) independently to about 100° F. (38° C.). Different, non-limiting threshold or trigger temperatures are about 90° F. (32° C.) independently to about 100° F. (38° C.). For all of these temperatures, it should be realized that these are initial temperatures and that it is expected that in most cases the temperature will rise further.

The method and formulation involve two phenomena: a) Pseudo-crosslinking of the elongated or “worm-like” VES micelles formed of the VES molecules, which helps the gelled fluid to fill the voids with its viscous flow and b) set like a cement plug (in the loss zone) over the period of time and elevated temperature. This is again a pseudo-crosslinking where VES forms elongated worm-like micelle structure in presence of monovalent and/or multivalent salts like magnesium chloride, calcium chloride, calcium bromide, potassium chloride, aluminum chloride, and mixtures of them, etc. This is not polymer and crosslinker based crosslinking. In one non-limiting theory or explanation, when the fluid containing the VES and the at least one monovalent or multivalent salt is heated at least one monovalent or multivalent salt associate, link, connect, or relate the VES micelles to one another thereby further increasing the viscosity of the fluid. This is somewhat analogous to the way crosslinkers connect various polymer chains and is sometimes called “pseudo-crosslinking”, but the way the at least one monovalent or multivalent salt associate the elongated or “worm-like” VES micelles is believed to be completely different than the crosslinking that occurs in polymers.

In the formulations and methods described herein, a magnesium powder (MgO) also works as cement which sets hard; and a boron-based compound works as a retarder to control that hardening/setting process with respect to temperature and time. Hardening and retarding chemistry is further explained below.

The viscoelastic surfactants suitable for use herein include, but are not necessarily limited to, non-ionic, cationic, amphoteric, and zwitterionic surfactants. Specific examples of zwitterionic/amphoteric surfactants include, but are not necessarily limited to, dihydroxyl alkyl glycinate, alkyl ampho acetate or propionate, alkyl betaine, alkyl amidopropyl betaine and alkylimino mono- or di-propionates derived from certain waxes, fats and oils. Quaternary amine surfactants are typically cationic, and the betaines are typically zwitterionic. The thickening agent may be used in conjunction with an inorganic water-soluble salt or organic additive such as phthalic acid, salicylic acid or their salts.

Some non-ionic fluids are inherently less damaging to the producing formations than cationic fluid types, and are more efficacious per pound than anionic gelling agents. Amine oxide viscoelastic surfactants have the potential to offer more gelling power per pound, making it less expensive than other fluids of this type.

The amine oxide gelling agents RN⁺(R′)₂O⁻ may have the following structure (I):

where R is an alkyl or alkylamido group averaging from about 8 to 24 carbon atoms and R′ are independently alkyl groups averaging from about 1 to 6 carbon atoms. In one non-limiting embodiment, R is an alkyl or alkylamido group averaging from about 8 to 16 carbon atoms and R′ are independently alkyl groups averaging from about 2 to 3 carbon atoms.

One specific suitable VES is ARMOVIS EHS® VES surfactant supplied by Akzo Nobel. The structure of which is shown in formula (II). This VES will help to build the viscosity for higher temperature up to 350° F. (177° C.) in presence of monovalent and/or multivalent salt brine.

Compound name ARMOVIS EHS: Erucamidopropyl Hydroxypropylsultaine

Typical properties of ARMOVIS EHS® VES surfactant include the following:

-   -   Appearance—liquid, with 50% solids     -   Pour point—41° F. (12° C.)     -   pH—8     -   Specific gravity—1.0     -   Solvent package—EtOH/PG/Water

As mentioned, the monovalent salts and/or multivalent salts serve as pseudo-crosslinking agents, which further increase entanglement of the elongated micelles and hence increase viscosity further. FIG. 7 is a chart of a viscosity comparison of fluid by HPHT 5550 Viscometer with and without VES illustrating that the presence of VES can greatly increase viscosity until such time as the viscosity is broken. FIG. 8 is a photograph of a solid set after a thickening time test for a 15 ppg (1.8 kg/L) slurry.

Another suitable specific VES is an amine oxide of formula (I) namely AROMOX® APA TW supplied by Akzo Nobel that may be used for the lower temperature range to build the pseudo-crosslinking up to 250° F. (121° C.).

The amount of the VES with respect to the water in the aqueous fluid may range from about 0.01 wt % independently to about 10 wt %; about 0.01 gps independently to about 2 gps. As used herein with respect to a range, “independently” means that any lower threshold may be combined with any upper threshold to give a suitable alternative range. Stated another way, the at least one VES is present in the aqueous fluid in an amount from about 0.01 gps independently to about 2 gps; alternatively from about 0.05 gps independently to about 1 gps. It will be appreciated that the abbreviation “gps” also refers to the cement-forming aqueous fluid described herein as a basis.

Suitable monovalent salts, divalent salts, or multivalent salts include, but are not necessarily limited to, sodium chloride, potassium chloride, ammonium chloride, sodium bromide, sodium formate, potassium formate, calcium chloride, calcium bromide, magnesium chloride, zinc chloride, zinc bromide, aluminum chloride, saline water, and combinations thereof. To be clear, in some non-limiting embodiments, simply using saline water will provide enough monovalent salts or multivalent salts to pseudo-crosslink the elongated VES micelles.

With respect to suitable amounts of the at least one monovalent or multivalent salt, the amount of the at least one monovalent or multivalent salt is present in the aqueous fluid in an amount from about 0.01 independently to about 10% bwoc. Magnesium chloride salt up to 10% bwoc has been successfully used depending upon application, but salts can also be used up to saturation weight. An alternative range may be from about 1 independently to about 10% bwoc.

The sealant powder or magnesium powder reacts in the presence of water to form magnesium oxysulfate cement according to formula (III) below:

5MgO+MgSO₄+8H₂O→5Mg(OH)₂.MgSO₄.3H₂O magnesium oxysulfate  (III)

Magnesium powder is primarily a blend of MgO and dolomite. Dolomite is an anhydrous carbonate material composed of calcium magnesium carbonate having the chemical formula CaMg(CO₃)₂. In one non-limiting embodiment the magnesium powder has 30-80 wt % MgO and greater than 20 wt % dolomite. In one non-limiting embodiment the amount of the at least one magnesium powder present in the cement-forming aqueous fluid or the cement is an amount up to about 100 wt %.

Magnesium oxysulfate cement formation is controlled by sodium borate retarder. Sodium borate reacts with MgO and MgSO₄ to form magnesium borate and thus inhibit the formation of magnesium oxysulfate. The sealant (magnesium) powder can be used to design 10 ppg to 14 ppg fluid (1.2 to 1.7 kg/L) as well as higher density fluids; this is used as 100% by weight of cement (bwoc). Basically fluid density of water 8.342 ppg (1 kg/L) and sealant powder are used to increase the weight up to the desired density. MgO powder is limited to use up to 230° F. (110° C.). FIGS. 1D and 8 are photographs of a resulting hard cement plug of magnesium oxysulfate.

Suitable retarders include, but are not necessarily limited to, sodium borate, boric acid, disodium tetraborate decahydrate, and combinations thereof. The retarder is also sometimes called boron powder. As noted, the retarder is used to delay the thickening time of this magnesium powder based cement. It has been discovered that conventional retarders such as lignosulfonate, phosphate based synthetic retarder for Portland cement will not work here due to the different hydration and setting chemistry. Suitable proportions for the boron-based powder or retarder to be used in the fluid range from about 0.01% independently to about 20% bwoc and above can be used; alternatively, the proportion may range from about 1% independently to about 15% bwoc.

The aqueous fluid may also optionally include a defoamer as necessary to control excessive foaming that may interfere with the fluid loss material placement. Suitable defoamers include, but are not necessarily limited to, silicon-based defoamers, such as silicon emulsions and the like. It was discovered that alcohol-based defoamers do not work with this system. Typical concentration ranges for the defoamer is from about 0.001 gps independently to about 0.1 gps; alternatively from about 0.005 gps independently to about 0.05 gps.

The abbreviation “gps” is for Gallon per Sack of Cement (in the case of the cement-forming aqueous fluid compositions described herein, the magnesium powder). One sack of this cement contain 55 pound (lb.). All the calculations and concentrations of the additives/ingredients are based on the amount of cement while cement is taken as 100% in the formulation to design cement slurry. The approximate density range of the cement slurry of the aqueous fluid system ranges from about 10 ppg to about 16 ppg (pounds per gallon) (1.2 to 1.9 kg/L) and higher, as will be explained below.

The initial gel of the aqueous fluid begins to build up at about 80° F. (27° C.); alternatively at about 100° F. (38° C.) and the system can work up to as high as about 300° F. (149° C.).

As noted, an advantage of the gel cement system described herein is that after the need for the hard cement plug is over, it may be dissolved with an acid flush in a conventional manner. Suitable acids for the flush include, but are not necessarily limited to, hydrochloric acid, formic acid, acetic acid, nitric acid, methane sulfonic acid, glutaric acid, glutamic acid, succinic acid, adipic acid, oxalic acid, glycolic acid, lactic acid, aminopolycarboxylic acids, and mixtures thereof.

In one embodiment, the methods and compositions herein are practiced in the absence of gel-forming polymers and/or gels or aqueous fluids having their viscosities enhanced by polymers. A known difficulty with polymers is that if they form a filter cake that penetrates the formation, the cake is difficult to remove without permanently damaging the near wellbore region of the formation. However, combination use with polymers and polymer breakers may also be of utility. For instance, polymers may also be added to the VES fluid for further improvement of fluid loss control. Types of polymers that may serve as fluid loss control agents include, but are not necessarily limited to, various starches, modified starches, polyvinyl acetates, polylactic acids, guar and other polysaccharides, hydroxyethylcellulose and other derivatized celluloses, gelatins, and the like.

The aqueous fluids and methods as described herein may be used in conjunction with other technologies including, but not necessarily limited to, U.S. Pat. Nos. 8,544,565 and 8,921,285, incorporated herein by reference in their entirety.

In some circumstances, formation pressure requires a heavier, denser mud to balance against this higher pressure. For mud densities in excess of about 14 pounds per gallon (1.7 kg/L), weighting materials must be added to the mud. Thus, particulate weighting materials including barium sulfate, micronized barite, hausmannite, ilmenite, manganese tetroxide, hematite, as non-limiting embodiments, and other fine, dense particles are added to the mud to increase its density. When such weighting materials are used in addition to the VES gelled MgO based cement fluid formulation, dispersants, such as sulfonated copolymers, having a molecular weight between about 2,000 independently to about 200,000, alternatively between about 5,000 independently to about 20,000, used with 0.05 to 0.5 wt % must be added to the fluid formulation to uniformly disperse the solid weighting agents and to temporarily reduce fluid viscosity for easier pumping. The weighting agents and dispersants do not reduce final fluid performance for loss circulation control. Available dispersants such as lignosulfonate, acetone formaldehyde, polyacrylamide cannot be used with a pseudo crosslink fluid of magnesium oxide based cement as these dispersants break the pseudo-crosslink structure. A sulfonated copolymer-based dispersant may be used in these weighted formulations with no effect on the pseudo-crosslinked magnesium powder based cement.

The present invention will be explained in further detail in the following non-limiting Examples that are only designed to additionally illustrate the invention but not narrow the scope thereof.

Laboratory Mixing Procedure for the Loss Control Material

The basic loss control material (LCM) used in conjunction with weighting materials to form a denser mud is the same as that described in the co-pending application U.S. Ser. No. 14/729,567, filed Jun. 3, 2015, incorporated herein by reference in its entirety. The fluid loss control material is mixed in a bottom drive blade type mixer used in the preparation of fluid slurries, as follows:

-   -   Fill the required amount of fresh water or saline water and set         the speed to 1000 rpm.     -   Add the monovalent and/or multivalent salt and retarder and mix         for 2-5 minutes.     -   Add the VES surfactant and mix for 10-15 minutes.     -   Add defoamer if required.     -   Add the sealant powder, weighting agent and dispersant at         2000-4000 rpm. Let the mixture become uniform.

Shown in FIGS. 1A-1D (FIG. 1) are top view photographs of mixing the components of a cement-forming aqueous fluid in a blender container, and the opening of a vortex after addition of the dispersant. More specifically, FIG. 1A is a photograph of a VES aqueous fluid containing MgO-based sealant powder in the blender with no dispersant and showing no vortex. FIG. 1B is a photograph of the VES aqueous fluid containing MgO-based sealant powder in the blender of FIG. 1A during dispersant addition. FIG. 1C is a photograph of the VES aqueous fluid containing MgO-based sealant powder in the blender of FIG. 1B with dispersant during blending showing that a vortex opens after dispersant addition. The presence of a vortex is indicative of mixability and pumpability. FIG. 1D is a photograph of the final set loss control material (LCM) plug made from the components mixed in FIGS. 1A-1C after being cured and removed from a mold.

Test Results Formulations Having a Density Greater than 14 PPG (1.7 kg/L)

For required fluid density greater than 14 ppg (1.7 kg/L) independently up to the upper limit of 20 ppg (2.4 kg/L) fluid density, weighting agents, such as barium sulfate, micronized barite (<6 micron with about 2 micron average particle size), hausmannite, ilmenite, manganese tetroxide, hematite, calcium carbonate, dolomite, magnetite, and combinations of these and other fine, dense particles, are usually added into fluid to increase density. Weighting materials may range from about 1% independently to about 150% bwoc, alternatively about 3% independently to about 50% bwoc, and in a different non-limiting embodiment from about 5% to about 40% bwoc, as required to achieve the desired density.

In one non-limiting embodiment, barite, which is not acid soluble, may be included in the formulation in proportions of about 10 to about 30% bwoc to give slurries of about 15 to about 16 ppg (about 1.8 kg/L to about 1.9 kg/L). These MgO-based cements are acid-soluble, however.

In one experiment MICROMAX, which is manganese tetroxide, was used. It is an inert mineral red brown powder in color and denser than barite having 4.8 gm/cm³ density and has an average particle size of 0.5 micron. MICROMAX with MgO based cement set system are acid soluble above 90% with 15% HCl. Table I provides data for an acid solubility test for MICROMAX for 16 ppg (1.9 kg/L) 2″×2″ (5.1×5.1 cm) cube with MICROMAX (manganese tetroxide) weighing agent in 15% HCl, where 93.1% acid solubility was observed. The formulation was also tested with MgO based cement for 12 and 14 ppg (1.4 and 1.7 kg/L).

TABLE I ACID SOLUBILITY WITH MICROMAX 12.5 ppg 14 ppg 16 ppg Acid Solubility 2″ × 2″ Cube 2″ × 2″ Cube 2″ × 2″ Cube Initial weight of the 144.58 215.4 292 cube (gm) Acid used (15% HCl) 800.46 1200 2000 (gm) Time to dissolve (hrs) 4 4 with stirring 5 with stirring Residue left (gm) 4.93 15.07 20.1 Solubility (%) 96.6% 93% 93.1%

For a VES gelled MgO based cement fluid formulation, dispersants, such as sulfonated copolymers, in a non-limiting example those having molecular weight of 10,000, used in an amount of from about 0.05 to about 0.5% wt are required to be added into the fluid formulation to help uniformly disperse the solid weighting agents and to temporarily reduce fluid's viscosity for easier pumping. The weighting agents and dispersants do not reduce final fluid performance for loss circulation control. Available dispersants such as lignosulfonate, acetone formaldehyde, polyacrylamide will not work with the pseudo crosslinked fluid with magnesium oxide based cement because these dispersants break the pseudo crosslink structure completely.

One suitable non-limiting sulfonated copolymer dispersant is a copolymer of sulfonic acid and monomers shown the structural formula below:

where: U=2-propenoic acid; V=benzene sulfonic acid 4-[(2-methyl-2 propenyl)oxy], sodium salt; w=2-propene-1-sulfonic acid, 2 methyl-, sodium salt; x=2-propenoic acid, 2-methyl-, methyl ester. The sulfonated monomers combine hydrophobic character, aromatic structure with weight average molecular weight between about 2,000 independently to about 200,000; alternatively from about 5,000 independently to about 15,000 and in one non-limiting embodiment 10,000. The sulfonated copolymer dispersant is added to the weighted formulation in the range of about 0.01% independently to about 2% bwoc; alternative from about 0.05 independently to about 0.5% bwoc. A suitable non-limiting specific example of a sulfonated copolymer is AQUATREAT AR=540, which is a copolymer of sulfonic acid and other monomers with a weight average molecular weight of 10,000. Another low weight average molecular weight polymeric dispersant suitable for use is NARALEX® D72, supplied by Akzo Nobel, which is a copolymer having sulfonate and carboxylate groups.

A sulfonated copolymer based dispersant is suitable for the crosslink magnesium powder based cement. Table II illustrates formulation examples of a 12.5 ppg slurry (1.5 kg/L) without dispersant and a 15 ppg fluid (1.8 kg/L) that includes dispersing agents. Lab testing data shows that 0.3% bw sulfonate copolymer can temporarily reduce fluid viscosity over 50% and has a compressive strength of greater than 300 psi up to 250° F. (121° C.) with acid solubility.

TABLE II 12.5 PPG AND 15 PPG FLUID DESIGN Additive gps & bwoc gps & bwoc Fluid density 15 ppg (1.8 kg/L) 12.5 ppg (1.5 kg/L) Fresh Water 3.06 gps 6.23 gps Defoamer 0.01 gps 0.01 gps Retarder 3% bwoc 3% bwoc Salt 1% bwoc 1% bwoc VES 0.06 gps 0.25 gps Sealant Powder 100% bwoc 100% bwoc Dispersant 0.3 Not needed

Table III presents the performance of various dispersants in a 15 ppg (1.8 kg/L) weighted formulation. The Table shows that without a dispersant, the fluid including the pseudo-crosslinked loss control material is not mixable. The Table also shows that the pseudo crosslinked structure of the loss control material does not persist with commonly used weighting material dispersants such as lignosulfonate, acrylamide and naphthalene formaldehyde- or acetone formaldehyde-based dispersants while sulfonated co-polymer dispersants maintain the pseudo crosslinking.

TABLE III RHEOLOGY OF LCM - 15 PPG (1.8 kg/L) WITH DIFFERENT DISPERSANTS Naphthalene Acetone Sulfonated formaldehyde formaldehyde Base Copolymer based based Lignosulfonate Acrylamide RPM  3 Not 40 25 27 25 Not Mixable  6 Mixable 50 35 50 35 100 175 120 135 110 200 130 150 160 140 300 125 170 180 160 600 155 240 290 255 10 s 60 27 30 26  1 min 72 28 30 30 10 min 97 30 35 35 Mixing observation No Yes Yes Yes Yes No Dispersant 0.3 0.3 0.3 0.3 0.3-0.5 concentration (% wt) Cross-linked persist Yes No No No No

Table IV shows exemplary formulations for achieving 15 ppg (1.8 kg/L) and 16 ppg (1.9 kg/L) fluid densities in accordance with the methods and compositions described herein.

TABLE IV Formulations for Achieving 15 ppg (1.8 kg/L) and 16 ppg (1.9 kg/L) Fluid Densities Additive 15 ppg (gps & bwoc) 16 ppg (gps & bwoc) Fresh Water 3.54 gps 3.6 gps Defoamer 0.01 gps 0.01 gps Retarder 10% bwoc 5% bwoc Salt 0.53% bwoc 0.5% bwoc Surfactant 0.07 gps 0.06 gps Sealant Powder 100% bwoc 100% bwoc Weighting Agent 10% bwoc 30% bwoc Dispersant 0.5% bwoc 0.5% bwoc

TABLE V and FIGS. 3-6, show the thickening times and compressive strengths for the 15 ppg and 16 ppg formulations at test temperatures of 125° F. (52° C.) and 200° F. (93° C.).

TABLE V Thickening Times and Compressive Strength Results for 15 ppg (1.8 kg/L) and 16 ppg (1.9 kg/L) Fluids Designs 15 PPG 16 PPG Test Temperature, 125° F. 200° F. 125° F. 200° F. F. (C.) (52° C.) (93° C.) (52° C.) (93° C.) TT (hr:min) 1:26 1:54 2:08 1:03 UCA, psi (MPa) 2202 (15.2) 784 (5.4) 2153 (14.8) 1179 (8.11)

A method and aqueous cement-producing fluid are provided as a loss circulation solution. An important differentiation over the use of only viscoelastic surfactants (VES) or similar chemistry that the aqueous fluid herein enables the formulation to be pumpable and to defer gelation until the fluid it is in right place in the formation. As noted, the fluid contains viscoelastic surfactant or similar chemistry with one or more soluble monovalent salt or brine and/or one or more multivalent salt or brine. For loss circulation prevention or inhibition, it is important to avoid premature gelation and for the gelled material to have sufficient viscosity and strength. Current commercial offerings have insufficient operational robustness; examples being commercial LCM technology based on polymer resin sealing, thixotropic slurries or high fluid loss pills.

The present methods and aqueous fluids provide a better operating window. Further advantages include: (a) it can be formulated with saline water and (b) it could be cleaned up post-job to recover a pre-job reservoir state—that is, with little or no damage to the reservoir. Use of VES, with or without saline water, builds enough pumpable viscosity that carries powder mix of active ingredients, namely the magnesium-based powder used for desired density and borate powder used as a retarder for control the solid cement set. Different weighting agents, such as barite, micronized barite, hausmannite, hematite, ilmenite, combinations of these and other dense fine particles can be used with MgO based powder to increase density. The choice of magnesium-based powder is also important to the functioning of this formulation. The formulation does not falsely setup in ambient conditions. The active mix is pumped to the potential fluid loss zones in the reservoir and non-reservoir portions of the subterranean formation. Upon being subjected to the wellbore temperature, it sets up as a cementitious plug with adequate strength to prevent and/or inhibit the fluid losses. At the end of the procedure, if required, the set solid material can be dissolved with acid solubility.

In the foregoing specification, the invention has been described with reference to specific embodiments thereof, and has been demonstrated as effective in providing methods and compositions for using VES-gelled aqueous fluids comprising powders to inhibit or prevent fluid loss. However, it will be evident that various modifications and changes can be made thereto without departing from the broader scope of the invention as set forth in the appended claims. Accordingly, the specification is to be regarded in an illustrative rather than a restrictive sense. For example, specific combinations of viscoelastic surfactants, brines, monovalent salts, divalent salts, multivalent salts, magnesium powders, retarders, weighting agents, dispersants, and optional defoamers and other components falling within the claimed parameters, but not specifically identified or tried in a particular method or aqueous fluid, are anticipated to be within the scope of this invention. Similarly, it is expected that the fluid loss prevention and inhibition methods may be successfully practiced using somewhat different mixing methods, temperature ranges, and proportions than those described or exemplified herein.

The words “comprising” and “comprises” as used throughout the claims is interpreted “including but not limited to”.

The present invention may suitably comprise, consist of or consist essentially of the elements disclosed and may be practiced in the absence of an element not disclosed. For instance, there may be provided a method of drilling into a subterranean formation comprising, consisting essentially of, or consists of, introducing into a wellbore passing at least partially through the subterranean formation a cement-forming aqueous fluid having a density greater than 14 pounds per gallon (ppg) (1.7 kg/L), where the cement-forming aqueous fluid consists essentially of or consists of water, at least one VES, at least one monovalent or multivalent salt, at least one magnesium powder 30-80 wt % MgO and greater than 20 wt % dolomite; at least one retarder, at least one weighting material or agent, at least one dispersant, and optionally a defoamer. The method further consists essentially of or consists of increasing the viscosity of the cement-forming aqueous fluid by the action of the at least one VES forming elongated micelles; where the at least one monovalent salt is present in an amount effective to pseudo-crosslink the elongated VES micelles to further increase the viscosity of the aqueous fluid, forming a cement by reacting at least one magnesium powder and the water, where the forming of cement is retarded from that which would otherwise occur but for the presence of the retarder, and inhibiting fluid loss of the fluid into the formation by the combined action of the pseudo-crosslinked VES micelles and the cement.

There is further provided a cement-forming aqueous fluid having a density greater than 14 ppg (1.7 kg/L) for use in inhibiting the fluid loss of the aqueous fluid into a subterranean formation, used for cement zonal isolation, the aqueous fluid consisting essentially of or consisting of water, at least one VES, at least one monovalent or multivalent salt, at least one magnesium powder comprising 30-80 wt % MgO and greater than 20 wt % dolomite, at least one retarder, at least one weighting material or agent, at least one dispersing agent, and optionally a defoamer. 

What is claimed is:
 1. A method of drilling into a subterranean formation comprising: introducing into a wellbore passing at least partially through the subterranean formation a cement-forming aqueous fluid having a density greater than 14 pounds per gallon (ppg) (1.7 kg/L) comprising: water; at least one viscoelastic surfactant (VES); at least one monovalent or multivalent salt; at least one magnesium powder comprising: 30-80 wt % MgO, and greater than 20 wt % dolomite, which magnesium powder works as cement; at least one retarder; at least one weighting material; and at least one dispersing agent; increasing the viscosity of the cement-forming aqueous fluid by the action of the at least one VES forming elongated micelles; where the at least one monovalent salt or multivalent salt is present in an amount effective to pseudo-crosslink the elongated VES micelles to further increase the viscosity of the aqueous fluid; forming a cement by reacting the at least one magnesium powder and the water, where the forming of cement is retarded from that which would otherwise occur but for the presence of the retarder; and inhibiting fluid loss of the fluid into the formation by the combined action of the pseudo-crosslinked VES micelles and the cement.
 2. The method of claim 1 where the weighting material is selected from barium sulfate, micronized barite, hausmannite, ilmenite, manganese tetroxide, hematite, calcium carbonate, magnetite, and combinations of these.
 3. The method of claim 2 wherein the weighting material is present in an amount of about 1% to 150% bwoc.
 4. The method of claim 1 where the dispersing agent comprises a sulfonated copolymer.
 5. The method of claim 4 wherein the sulfonated copolymer is present in an amount of about 0.01% to 2% bwoc.
 6. The method of claim 1 where the at least one VES is present in the cement-forming aqueous fluid in a range of from about 0.01 wt % to about 10 wt % based on the water.
 7. The method of claim 1 where the at least one VES is present in the cement-forming aqueous fluid in an amount from about 0.01 gps to 2 gps.
 8. The method of claim 1 where the at least one monovalent or multivalent salt is selected from the group consisting of sodium chloride, potassium chloride, ammonium chloride, sodium bromide, sodium formate, potassium formate, calcium chloride, calcium bromide, magnesium chloride, zinc chloride, zinc bromide, aluminum chloride, and combinations thereof.
 9. The method of claim 8 where an amount of the at least one monovalent or multivalent salt is present in the water in an amount from about 0.05 bwoc to above saturation.
 10. The method of claim 1 where the at least one retarder is selected from the group consisting of sodium borate, boric acid, disodium tetraborate decahydrate, and combinations thereof.
 11. The method of claim 1 where the at least one retarder is present in the aqueous fluid in an amount from about 0.01 to about 20% bwoc.
 12. The method of claim 1 where the cement has an acid solubility greater than 90%
 13. The method of claim 1 where the cement has a compressive strength greater than 300 psi.
 14. A method of drilling into a subterranean formation comprising: introducing into a wellbore passing at least partially through the subterranean formation a cement-forming aqueous fluid having a density greater than 14 pounds per gallon (ppg) (1.7 kg/L) comprising: water; at least one viscoelastic surfactant (VES); at least one monovalent or multivalent salt selected from the group consisting of sodium chloride, potassium chloride, ammonium chloride, sodium bromide, sodium formate, potassium formate, calcium chloride, calcium bromide, magnesium chloride, zinc chloride, zinc bromide, aluminum chloride, and combinations thereof; at least one magnesium powder comprising: 30-80 wt % MgO, and greater than 20 wt % dolomite, which magnesium powder works as cement; at least one retarder; at least one weighting material selected from barium sulfate, micronized barite, hausmannite, ilmenite, manganese tetroxide, hematite, calcium carbonate, magnetite, and combinations of these; and at least one dispersing agent; increasing the viscosity of the cement-forming aqueous fluid by the action of the at least one VES forming elongated micelles; where the at least one monovalent salt or multivalent salt is present in an amount effective to pseudo-crosslink the elongated VES micelles to further increase the viscosity of the aqueous fluid; forming a cement by reacting the at least one magnesium powder and the water, where the forming of cement is retarded from that which would otherwise occur but for the presence of the retarder; and inhibiting fluid loss of the fluid into the formation by the combined action of the pseudo-crosslinked VES micelles and the cement.
 15. The method of claim 14 wherein the weighting material is present in an amount of about 1% to 150% bwoc.
 16. The method of claim 14 where the at least one VES is present in the cement-forming aqueous fluid in a range of from about 0.01 wt % to about 10 wt % based on the water.
 17. The method of claim 14 where the at least one VES is present in the cement-forming aqueous fluid in an amount from about 0.01 gps to 2 gps.
 18. The method of claim 17 where an amount of the at least one monovalent or multivalent salt is present in the water in an amount from about 0.05 bwoc to above saturation.
 19. The method of claim 14 where the at least one retarder is present in the aqueous fluid in an amount from about 0.01 to about 20% bwoc.
 20. A method of drilling into a subterranean formation comprising: introducing into a wellbore passing at least partially through the subterranean formation a cement-forming aqueous fluid having a density greater than 14 pounds per gallon (ppg) (1.7 kg/L) comprising: water; at least one viscoelastic surfactant (VES), at least one monovalent or multivalent salt selected from the group consisting of sodium chloride, potassium chloride, ammonium chloride, sodium bromide, sodium formate, potassium formate, calcium chloride, calcium bromide, magnesium chloride, zinc chloride, zinc bromide, aluminum chloride, and combinations thereof, where an amount of the at least one monovalent or multivalent salt is from about 0.05 bwoc to above saturation; at least one magnesium powder comprising: 30-80 wt % MgO, and greater than 20 wt % dolomite, which magnesium powder works as cement; at least one retarder present in an amount from about 0.01 to about 20% bwoc; at least one weighting material present in an amount of about 1% to 150% bwoc, where the at least one weighting material is selected from barium sulfate, micronized barite, hausmannite, ilmenite, manganese tetroxide, hematite, calcium carbonate, magnetite, and combinations of these; and at least one dispersing agent present in an amount of about 0.01% to 2% bwoc; increasing the viscosity of the cement-forming aqueous fluid by the action of the at least one VES forming elongated micelles; where the at least one monovalent salt or multivalent salt is present in an amount effective to pseudo-crosslink the elongated VES micelles to further increase the viscosity of the aqueous fluid; forming a cement by reacting the at least one magnesium powder and the water, where the forming of cement is retarded from that which would otherwise occur but for the presence of the retarder; and inhibiting fluid loss of the fluid into the formation by the combined action of the pseudo-crosslinked VES micelles and the cement. 